(87e) Modeling Fluidized Beds for Application to Biomass Systems

The use of gasification to produce biorenewable forms of energy has recently regained popularity [1-2]. This process is typically carried out in fluidized beds as they offer advantages such as low pressure drop, approximately uniform temperature distributions, high heat and mass transfer, and the ability to fluidize many particle types of various sizes. Biomass is injected into a heated bed of inert material, such as sand, and undergoes gasification, the result of which is a flammable hydrocarbon gas. This product can be used on site or can be further processed, for example through Fisher-Tropsch synthesis.

Improving gasification efficiency in fluidized beds requires a detailed understanding of the fluidization behavior. Computational Fluid Dynamics (CFD) modeling is an important tool for improving our understanding, however extensive validation of CFD models is required. In fact, when biomass is modeled, several required model inputs are not known or not easily measured experimentally [3].

The aim of this work is to validate CFD models applied to single component biomass systems against experimental data obtained by means of X-ray Computed Tomography (CT). An important result of the validation process is the needed use of an ?effective density? for biomass in the CFD model. In fact, if the mid-value of the density range provided by the manufacturer is employed, the bulk density of the solid phase in the bed is overestimated, because the irregular particle shape and particle porosity are not included in the CFD model, which treats biomass particles as solid spheres.

In previous work [4] on the same fluidized bed facility, the validation of CFD models applied to glass beads systems showed that qualitative agreement between experiments and simulations was very good and quantitative bed pressure drop comparisons were also excellent. Measured local time-averaged gas fractions were also compared to simulated values with the overall agreement being satisfactory.

The present work employs ground walnut shell or ground corncob as model biomass bed material, and fluidization is performed at a gas velocity twice the minimum fluidization velocity (Ug = 2 Umf). Simulations are run on the computational grid and with the drag model (Syamlal-O'Brien) employed in [4]. In order to tune the computational model for specific application to biomass, several combinations of particle sphericity (φ) and coefficient of restitution (COR) are tested. It is observed that the bed height is well predicted with COR = 0.9 for φ = 0.8--1. For smaller φ, the predicted bed height is higher, consistent with the results of Deza et al. [3]. The results also suggest that the value of COR has a negligible effect on the predictions for biomass systems.

References

1. Bridgewater, A.V., ?Renewable fuels and chemicals by thermal processing of biomass?, Chemical Engineering Journal, 91: 87-102 (2003).

2. Cui, H., and Grace, J.R., ?Fluidization of biomass particles: a review of experimental multiphase flow aspects?, Chemical Engineering Science, 62: 45-55 (2007).

3. Deza, M., Battaglia, F., and Heindel, T.J., ?A Validation Study for the Hydrodynamics of Biomass in a Fluidized Bed? in Proceedings of FEDSM2008: 2008 ASME Fluids Engineering Division Summer Conference, Jacksonville, FL: ASME Press, Paper FEDSM2008-55158 (2008).

4. Min, J., Drake, J.B., Heindel, T.J., Fox, R.O., ?Experimental validation of CFD simulations of a lab-scale fluidized bed reactor with and without side-gas injection?, AIChE Journal, submitted (2008).